Methods of selective carburization of ferrous metal surfaces and materials therefor



United States Patent METHDDS 0F SELECTEVE CARBURIZATION 0F FERROUS METAL SURFACES AND MATERIALS THEREFOR Willi Benz and Kurt Ego, Friedrichshafen, Germany,

assignors to Zahnradfabrik Friedrichshafen, Aktiengeselischaft, Friedricnshafen, Germany No Drawing. Filed Jan. 1d, 1960, Ser. No. 2,834 Claims priority, application Germany Sept. 26, 1959 6 Claims. (Cl. 148-165) This invention relates to protective coatings or so called resist coatings to be applied to the surfaces of iron and steel parts prior to carburization in order to protect predetermined surfaces against carburization.

In general, the carburization by means of gases and powders of the ferrous metal surfaces is well known, as are various methods and materials of protecting predetermined areas of such surfaces where carburization is not desired. Thus, the effect of carburization is to increase skin strength of a metal part against wear and abrasion by virtue of surface hardening of such part upon exposure to carburizing agents such as carburizing powders and carburizing, gaseous atmospheres. Under some circumstances, however, it is desired to prevent hardening of the surfaces of selected areas of machine and structural components. Various materials and methods for excluding selected surface areas from the hardening effect have been utilized. Generally the application or covering of areas to remain uncarburized is by means of silicates and clay materials, galvanic plating, and the like. For example, borax or other alkaline earth borates mixed as a flux for fire clay or powdered sodium silicate has been used, wherein the flux agent is intended to prevent crumbling of the powdery clay or silicate ingredient at the high carburizing temperatures normally encountered.

The resist or protective coatings used in the prior art are, however, not well suited for use in gaseous-atmosphere types of carburization due to their high water content. Further, the application of such compounds is time consuming due to the fact that a special drying process is required. Finally, the prior art compounds cannot be put on selected surfaces readily, as by a spray gun.

It is an object of the present invention to provide methods and materials for selective carburization, wherein such materials are cheap and readily applied, as by dipping or by a spray gun.

It it a further object of the invention to provide methods and materials that will insure maximum resist eifect in protecting selected areas against carburization.

It is still another object of the invention to provide methods and materials which are safe and simple to use and-very rapid in application and in removal subsequent to the carburization process.

Our invention comprises the use of a resist or protective substance consisting of boric acid (H BO or boric acid anhydride (B 0 or either of these substances combined with binders, such as synthetic resin lacquer or alumina.

The use of boric acid as a protective agent against carburization is readily effected by mere submergence of preselected areas of a structural element or machine part into a boric acid solution, preferably a saturated solution at about 80 C. As thus applied, the boric acid crystalizes onto the areas immersed to form a crystalline deposit that serves as a protection against carburizing gases and powders to maintain such selected areas uncarburized. Instead of applying the boric acid crystalline coating by dipping. it may also be applied with a spray gun in accordance with conventional spray gun oper- BJELMZ Patented Sept. 29, 1964 ation, it being understood that the acid solution is preheated to the preferable C. temperature.

In applying the boric acid anhydride, the compound is heated beyond its melting point of about 580 C. and the selected areas to remain uncarburized are then coated with the melt by submergence. Such a coating has a viscous consistency at carburizing temperatures and adheres readily to the selected area to be protected. It will, of course, be understood that, in keeping with modern methods of spraying of hot compounds, for example molten metals, spraying of molten boric acid anhydride may also be used.

After selected areas have thus been protected in accordance with the above description, the ferrous metal element is passed through a carburization process, subsequent to which the coating of boric acid or boric acid anhydride is readily removed by hot water washing or sand blasting.

We have also found that protective compounds may be made of mixtures of boric acid and/or boric acid anhydrides, and anhydrous binding and filling agents, such as synthetic resin lacquer or alumina. Such compounds may be applied either by spraying, brushing, or dipping on either alloyed or unalloyed steels. In either case, the synthetic resin lacquer serves as a binder, and the alu mina, or a similar ingredient, serves as a filler.

The percentage proportion of boric acid or anhydride in ready-touse mixes depends on the ratio of boric acid or anhydride to the dry substance of the synthetic resin lacquer used, and can vary from 1-to-1 to 1-to-4 Without losing protective effect. Also, various thinning agents may be added, depending on whether the coating is to be applied by spraying, brushing, or dipping, and the proportion of boric acid or anhydride used can vary between very wide extremes, for example, from ten percent to percent in a ready-to-use mix.

By way of a practical example, for a ready-to-use mix, a composition may be comprised as follows:

50% of organic binder, for example, synthetic resin lacquers, at a viscosity of 63 seconds passage at 20 C. in the Fordglas-s and at a tuyere area of 2 mm.

40% boric acid anhydride with a grain size of less than 7% boric acid with a grain size of less than 6.1 mm.

3% alumina with a grain size of less than 0.1 mm.

The compound can be prepared by mixing the ingredients together without the use of grinding, and the mixed compound can be stored practically indefinitely.

In actual use, after the mixture has been applied to selected areas and the elements to be carburized put into a carburizing atmosphere or oven, the boric acid or anhydride heat up to form filling agents having a binding agent formed from the products of carburization of the organic binding agent which is destroyed during heating. Thus, a coking residue of the organic binding agent remains embedded in the melted boric acid or anhydride. The addition of substances such as alumina, which do not melt at carburization temperatures, is advantageous in the event of insutlicient coking residue of the organic binding agent.

Having thus described our invention, we are aware that various changes may be made without departing from the spirit thereof, and accordingly, do not seek to be limited to the precise description given, except as set forth in the appended claims.

We claim:

1. The method of selective carburization of the surface of a ferrous metal which comprises applying to such area as is not to be carburized a coating with a member selected from a group consisting of boric acid and boric acid anhydride, and an organic anhydrous binding agent, in effective amounts to form a resist having a coking resi- 9 due during carburization, then carburizing said metal at a temperature whereat said binding agent forms said coking residue, and subsequently removing said coating.

2. A method of selectively carburizing ferrous metal which comprises applying to an area thereof which is to remain uncarburized a coating of a composition comprising, in effective amounts to form a resist having a coking residue during carburization, a member selected from the group of boric acid and boric acid anhydride and an anhydrous organic binding agent, said composition being applied in molten form, then carburizing said metal, wherein said agent forms said coking residue at carburization temperature, and subsequently removing said coating.

3. The method of selective carburization of the surface of a ferrous metal which comprises applying to such area as is not to be carburized a coating with a member selected from a group consisting of boric acid and boric acid anhydride, and an organic anhydrous bind ing agent, to form a residue, then carburizing said metal at a temperature whereat said binding agent forms a coking residue, and subsequently removing said coating, wherein the ratio of the selected member to the binding agent is substantially within the range of 1:1 to 1:4.

4. A method of selectively carburizing ferrous metal which comprises applying to an area thereof which is to remain uncarburized a coating of a composition comprising a member selected from the group of boric acid and boric acid anhydride and an anhydrous organic binding agent, said composition being applied in molten form, then carburizing said metal, wherein said agent forms a coking residue at carburization temperature, and subsequently removing said coating, wherein the ratio of the selected member to the binding agent is substantially within the range 1:1 to 1:4.

5. The method of selective carburization of the sur- 4i face of a ferrous metal which comprises applying to such area as is not to be carburized a coating composition consisting essentially of a member selected from a group consisting of boric acid and boric acid anhydride, and an organic binding agent, in effective amounts to form a resist having a coking residue during carburization, then carburizing said metal anhydride at a temperature whereat said binding agent forms said coking residue, and subsequently removing said coating.

6. A method of selectively carburizing ferrous metal which comprises applying to an area thereof which is to remain uncarburized, a coating of a composition consisting essentially of a member selected from the group of boric acid and boric acid anhydride and an anhydrous organic binding agent, in effective amounts to form a resist having a coking residue during carburization, said composition being applied in molten form, then carburizing said metal at a temperature wherein said agent forms a coking residue at carburization temperature, and subsequently removing said coating.

References Cited in the file of this patent UNITED STATES PATENTS 1,190,937 Moore July 11*, 1916 1,567,632 Whinfrey Dec. 29, 1925 1,896,042 Ruben Jan. 31, 1933 1,896,043 Ruben Jan. 31, 1933 2,485,176 Waterfall Oct. 18, 1949 2,504,001 Connolly Apr. 11, 1950 2,584,405 West Feb. 5, 1952 2,898,253 Schneider et al Aug. 4, 1959 OTHER REFERENCES Revue de Metalurgie, vol. 19, 1922 (pp. 222-226). Protective Coatings for Metals, Burns and Schuh, published by Reinhold, 1939 (page 197). 

1. THE METHOD OF SELECTIVE CARBURIZATION OF THE SURFACE OF A FERROUS METAL WHICH COMPRISES APPLYING TO SUCH AREA AS IS NOT TO BE CARBURIZED A COATING WITH A MEMBER SELECTED FROM THE GROUP CONSISTING OF BORIC ACID AND BORIC ACID ANHYDRIDE, AND AN ORGANIC ANHYDROUS BINDING AGENT, IN EFFECTIVE AMOUNTS TO FORM A RESIST HAVING A COKING RESIDUE DURING CARBURIZATION, THEN CARBURIZING SAID METAL AT A TEMPERATURE WHEREAT SAID BINDING AGENT FORMS SAID COKING RESIDUE, AND SUBSEQUENTLY REMOVING SAID COATING. 